18.01.06 Cell cycle, mitosis, meiosis, recombination

Question 1

What are the broad stages of the cell cycle?

Answer 1

Quiescent/senescent state = G0
Interphase = G1, S, G2
Cell division = Meiosis/Mitosis
Cytokinesis

Question 2

Define mitosis.

Answer 2

Is part of the cell cycle in which chromosomes in the nucleus are separated into two identical sets of chromosomes contained within their own nucleus.

Accounts for approximately 20% of the cell cycle and is divided into stages corresponding to the completion of one set of activities and the start of the next.

Question 3

Describe G0.

Answer 3

A resting phase where the cell has left the cycle and has stopped dividing.

Question 4

Describe G1.

Answer 4

9-12 hours

Growth phase during which proteins and RNA are synthesised. Each chromosome exists as a single double stranded helix - at no point is DNA synthesised in this phase.

At the G1 checkpoint - the restriction point - the cell is committed to division and moves into the S phase.

Question 5

Describe S phase.

Answer 5

6-8 hours

DNA synthesis replicates the genetic material. Each chromosome now consists of two sister chromatids.

Each of the 46 chromosomes is duplicated by the cell.

Question 6

Describe G2.

Answer 6

Loosely coiled replicated chromosomes

Cell continues to grow. The G2 checkpoint ensures enough cytoplasmic materials necessary for mitosis and cytokinesis.

The cell souble checks the duplicated chromosomes for error, making any needed repairs

Question 7

What is the function of cell cycle checkpoints?

Answer 7

Regulation of the cell cycle involves processes crucial to the survival of a cell, including the detection and repair of genetic damage as well as the prevention of uncontrolled cell division.

Regulatory pathways that control the order and timing of the cell cycle transitions ensuring critical events are completed with high fidelity.

Question 8

What types of protein regulat the cell cycle?

Answer 8

The cell cycle is regulated by heterodimeric protein kinases composed of:

1) Cyclins - form the regulatory sybunit and have no catalytic activity
2) Cyclin-dependent kinases (CDKs) - inactive in the absence of a partner cyclin. Becomes the catalytic subunit of an activated heterodimer which phosphorylates target proteins to orchestrate coordinated entry into the next phase of the cell cycle.

Question 9

What is the difference in expression between CDKs and cyclins?

Answer 9

CDKs are constitutively expressed whereas Cyclins are synthesised at specific stages in response to various external stimuli / molecular signals.

Question 10

What are the three main cell cycle checkpoints?

Answer 10

1) G1/S (restriction) checkpoint
2) G2/M checpoint
3) Metaphase/Spindle checkpoint

Question 11

What occurs at the G1/S (restriction) checkpoint?

Answer 11

1) Cell growth enables foramtion of the CDC-cycling D
2) Phosphorylated retinoblastima protein
3) Relieves inhibition of E2F transcription factor
4) Cyclin E now expressed, binds to CDK2
5) Allows G1-S phase transition

Question 12

What occurs during the G2/M checkpoint?

Answer 12

1) CSK1 is activated by phosphorylation and dephsophorylation of specific amino acid residues by Cyclin-Activating Kinase (CAK) and the wee1 protein
2) ENable CDK!-cyclin B formation (aka MPF)
3) Allows G2-M phase transition

Question 13

What occurs during the metaphase/spindle checkpoint?

Answer 13

1) Chromosomes assemble of metaphase plate
2) Anaphase-promoting complex (APC) activated
3) Degrades cyclin B = MPF diassembly
4) Relieves inhibition of ‘separase’ = spindle cut
5) Sister chromatid separation = anaphase entyry.

Question 14

How do cytogeneticists manipulate the cell cycle in rder to produce high quality metaphase preparations?

Answer 14

Mitogens - used to induce division of resting cells e.g. PHA

SYnchronisation - inhibitors block cell cycle during S phase by slowing/stopping DNA synthesis (e.g. BrDu)

Block released after 16-22hours cell continue through G” together

Colcemid arrests the cell during mitosis by prevents spindle fibre apparatus fomration.

Question 15

What are some of the roles of p53?

Answer 15

Controlling progression through G1/S and G2/M checkpoints

DNA damage activates p53 which inhibits progression through the checkpoint.

Question 16

Define meiosis.

Answer 16

Specialised type of cell division that reduces the chromosome number by half.

DNA replication is followed by two rounds of cell division (meiosis I and meiosis II) to produce four potential daughter cells, each with half the number of chromosomes as the original parent cell. The daughter cells are not genetically identical to the parent cells (unlike mitosis).

Question 17

What the stages of prophase I?

Answer 17

1) Leptotene
2) Zygotene
3) Pachytene (early)
4) Pachytene (late)
5) Diploteene
6) Diakinesis

Question 18

Describe leptotene.

Answer 18

Nuclear chromatin begins to condense/become visible. Chromosomes are unpaired fine threads consisting of 2 tightly bound sister chromatids (AKA “A string with beads.”)

Question 19

Describe zygotene.

Answer 19

Maternal and paternal homologues pair to form bivalents- held together by formation of synaptonemal complex. In Males, X and Y condense to form sex vesicle. The 2 sets of sister chromatids is termed a bivalent and the connecting points are chiasmata.

Question 20

Describe pachytene (early).

Answer 20

All homologues have paired (synapsis). Bivalent at this stage is called a tetrad.

Question 21

Describe pachytene (late).

Answer 21

Chromosomes thicken, cross over and recombination of genetic material occurs. Two non-sister chromatids cross over, the other 2 remain unaltered. Approx 60 (sperm) to 90 (ova) crossover events occur per cell with at least one per chromosome arm.

Question 22

Describe diplotene.

Answer 22

Homologues start to separate (desynapsis) but are held together by chiasmata- may resemble cross-like appearance. Sex vesicle disappears and X and Y appear associated end to end. The small chiasma formed is enough to keep X/Y chromosomes paired on spindle during metaphase- resulting sperm cells will have either X or Y chromosome.

Question 23

Describe diakinesis.

Answer 23

Bivalents are more contracted. Nuclear envelope breaks down. Oocytes reach this stage at ovulation

Question 24

Describe the stages of metaphase, anaphase and telophase I.

Answer 24

Metaphase: Spindle formed, bivalents align along metaphase plate. Organisation critical to ensure one copy of each chromosome us received in each new nucleus. In females, spindle is off centre and one resultant cell will contain more cytoplasm. X and Y may separate to form univalents

Anaphase I: Homologous chromosomes drawn apart. Chromatids remain together

Telophase I: Chromosomes at poles. Haploid daughter cells formed. In females, the cell with larger amount of cytoplasm is called secondary oocyte and the smaller one is first polar body.

Question 25

Describe the stages of prophase/metaphase, anaphase and telophase II.

Answer 25

Prophase/Metaphase II: Cells pass directly from meiosis I to metaphase II with no real prophase II. Occurs immediately in males. In females it is co-ordinated with ovulation and fertilisation. Nuclear envelope breaks down, new spindle formed, chromsomes (consisting of 2 chromatids) align.

Anaphase II: Separation of centromeres, migration of sister chromatids to opposite poles

Telophase II: Further cell division occurs forming 2 haploid cells (4 in total). In secondary oocyte, spindle is again off centre resulting in a large cell (ovum) and second polar body. Two more polar bodies may arise from the first but the ovum is the only viable gamete.

Question 26

When does gametogenesis occur in males and females?

Answer 26

Males: at puberty (spermatogenesis)

Females: in utero (oogenesis)

Question 27

What occurs at MI and MII of meiosis?

Answer 27

MI (meiosis 1) – random independent assortment of chromosome pairs and crossover enables genetic recombination prior to separation

MII (meiosis 2) – separation of chromatids (to haploid state)

Independent assortment of maternal and paternal homologues in human cells creates the possibility of 2^23 (8.4 million) genetically different gametes

Recombination of genetic material via crossing over forms daughter cells that contain composites of both parental genes; therefore, the actual number of genetically different gametes is infinite.

Question 28

Briefly describe the stages involved in spermatogenesis.

Answer 28

1. Spermatogenesis begins at puberty in the testes of males and continues in the epithelial lining of seminiferous tubules.
2. Spermatogonia differentiate into primary spermatocytes which undergo meiotic division
3. Following meiosis I, two spermatocytes are produced with 22 duplicated chromosomes and an X or Y chromosome (also in duplicate). These separate during meiosis II to give rise to four haploid spermatids which mature into sperm cells.
4. Sperm cells are released into the lumen of the seminiferous tubules to the epididymis where they are stored and undergo further maturation.
5. Complete cytokinesis does not occur until this stage.

Question 29

Briefly describe the stages of oogenesis.

Answer 29

1. In the developing embryo, Primordial germ cells migrate to the forming gonads to become oogonia.
2. These cells proliferate by mitotic divisions before differentiating into primary oocytes.
3. Meiosis I begins and continues until it reaches meiotic prophase I (dictyate stage-prolonged diplotene) at 8 months gestation.
4. The developing egg can remain in this stage for years - protein synthesis is occurring; a coat and cortical granules are formed (important during fertilisation!)
5. Meiosis I resumes at the onset of sexual maturity (Due to Luteinising Hormone (LH) surge, release of oocyte from follicle). Chromosomes re-condense and the nuclear envelope breaks down. Homologues separate into two daughter nuclei and cytokinesis gives rise to a secondary oocyte and the FIRST (small) polar body.
6. Meisosis II is arrested at metaphase II. At ovulation, the arrested oocyte is released; this becomes the precursor for the egg or ovum which results after the 2nd meiotic division along with a second polar body. The egg retains its large size by asymmetric division of the cytoplasm; polar bodies eventually degenerate.
7. Meiosis II is completed if fertilisation occurs.

Question 30

Briefly describe the stages of fertilisation.

Answer 30

1. Once released, both the egg and sperm die within hours unless fertilised. Fertilisation activates the egg
2. The sperm cell that reaches the egg cell triggers the acrosome reaction, a process in which hydrolytic enzymes of the acrosome are released on the external surface of the zona pellucida (the protective layer that surrounds the egg cell).
3. A portion of this layer is digested by the acrosomal enzymes allowing the sperm cell to reach the plasma membrane of the egg cell.
4. At the moment that the sperm cell makes contact with the egg cell membrane a chemical alteration of this membrane occurs.
5. Enzymes secreted by exocytosis (cortical reaction) make the zona pellucida unable to bind to other sperm cells (zonal reaction) and other male gametes cannot enter the egg cell.
6. After fusion with the membrane, the sperm is drawn head first into the egg and becomes a zygote. The tail is lost.
7. The pro-nuclei (from egg and sperm) remain distinct until the membranes break down prior to the first mitotic division.
8. Embryogenesis begins developing the zygote.

Question 31

What is nondisjunction?

Answer 31

Paired homologues (meiosis I) or sister chromatids (meiosis II) fail to separate

Homologues fail to pair (non-conjugation)

Sister chromatids separate too early (pre-mature separation) and segregate to the same daughter cell independently Lead to aneuploid embryo.

Full chromosome set fails to separate at meiosis I gametes with 46 chromosomes triploid conception.

Question 32

What is the effect of nondisjunciton at meiosis I and II?

Answer 32

Meiosis I Non-disjunction:
Produces 3 different variant chromosomes or three alleles

Meiosis II Non-disjunction:
Produces 2 identical varint chromosomes, or 2 identical alleles, from the same parent plus one other

Question 33

When does recombination take place?

Answer 33

Prophase I of meiosis - homologous chromsomes line up in pairs and swap segments of DNA (crossing over)

Question 34

What is involved in the process of genetic recombination?

Answer 34

Genetic recombination is a complex process that involves

1. Alignment of two homologous DNA strands
2. Precise breakage of each strand
3. Equal exchange of DNA segments between the two strands
4. Sealing of the resultant recombined DNA molecules through the action of enzymes called ligases.

Question 35

Give a brief overview of the process of meiotic recombination.

Answer 35

1. Recombination is initiated by DNA double-strand breaks
2. Ends are rapidly processed to form long single-stranded tails with 3’-termini
3. Inter-homolog single-end invasions (SEI) by the single-stranded tails takes place
4. SEI results in double-Holliday junctions (DHJ) formation
5. DHJs are resolved into cross-over products

Question 36

What are the potential consequences of recombination?

Answer 36

1. NAHR
2. Single gene disorders (Mendelian disorder)
3. Contiguous gene disorders
4. Segmental aneuploidy (aneusomy) syndromes